What Size Central Air Conditioner Do I Need for My House? A Short Consumer's Guide From ASM

Tim K.

15:57, 25 October 2014

338350

ASM is a U.S. Veteran-Owned commercial and residential HVAC company, and we have built our reputation on giving straight, honest answers to peoples’ questions. In order for these articles to be useful, we base most of our posts on the questions that we get asked most often, and questions on which size central air conditioner to buy are frequent. Most often, we’re asked, “what size central air conditioner do I need for my house?” There are many factors in your home that affect the proper size of your central air conditioner, such as the height of your ceilings and the type and thickness of your insulation. It should come as no surprise that with most of these questions, HVAC guys have a tendency to go off on some technical rant about load calculations and R-values; at ASM, we’re usually no different. But what do terms like “tonnage” and “load calculation” actually mean?

I will also say that I have been told by many (mostly my wife) that I too need to speak in more easy-to-understand terms, and stop going off on technical tangents when I talk about these things. So, hopefully to your benefit, in this article we will figure out how to determine what size central air conditioner you need for your house in layman terms, and we’ll do it in two parts: first, we will take a quick look at how to get a general idea of what size central air conditioner is right for your home, and second, we will go more in-depth as to how to properly use load calculations to find the right sized central air conditioning system for you and your family.

Part I: A Short, Layman’s Guide to What Size Central Air Conditioner to Buy for Your House

This section addresses three quick ways to calculate the right sized central air conditioner for your home, without getting into the nitty gritty. It is for the layman who isn’t all that familiar with air conditioners or how they work. The first two ways are rule-of-thumb, and will give you a decent ballpark of what size AC you should be looking at. The last way in this section uses an automated online program to calculate a more precise AC size, and does all of the work for you; pick your poison. But first, let’s talk about some important terms that you should know.

What Different Sizes Does Your Central Air Conditioner Come In?

So there are a few different ways to get a good idea of what sized central air conditioner you’ll need. Two methods are pretty close at getting a ballpark AC size estimate, and the third way is most precise, but is done with an automated, online program.

Before figuring out what sized central air conditioner is right for your house, though, you’ll need to know what sizes they come in (keep in mind, this article refers to your air conditioner size, not the type of air conditioner). For information on different AC types, see:

Central air conditioners come in a variety of sizes, and the size they are measured in is called “tonnage.” Now contrary to what you might think, the tonnage of an AC unit is not actually based on its weight. A “ton” is a measure of an air conditioner’s ability to cool. One ton is the ability of your air conditioner to cool 12,000 BTUs (British Thermal Unit) in an hour.

Likewise, a “2-ton” central air conditioner is able to cool 24,000 BTUs per hour. So now you are likely asking, what is a BTU? A BTU is the amount of energy required to heat or cool one pound of water by one degree Fahrenheit. So a 1-ton air conditioner can cool 12,000 pounds of water by one degree every hour. That’s all it means, so don’t let your HVAC contractor tell you any differently!

The central air conditioners that are installed in your home range from 1.5-ton to 5-ton units and increase in half-ton increments (2-ton, 2.5-ton, 3-ton, etc…). Anything over 5-tons is generally considered a light commercial HVAC unit (not available for residential projects) and if your house requires an AC unit of this size, then you’ll need to install multiple AC units in tandem. What this means is that if your home needs 6-tons of cooling power for instance, then you’d need two, 3-ton units installed (instead of a single 6-ton unit).

This may seem like overkill, but the nice part about two systems running together is that if one unit completely fails, then you still have one unit running. This might not keep your house at a comfortable 68-degrees but it can at least keep your house from sky-rocketing into the 90’s in the middle of summer. In general, unless you have a mansion or zero insulation in your home then you are unlikely to need anything more than a 5-ton unit. If you want to learn more about BTUs, take a look at Wikipedia’s page on BTU’s:

Why Does the Right Sized Central Air Conditioner Matter So Much?

Simply put, size matters. At the risk of alienating half of my readers, I am going to skip the obvious joke. It is a common misconception that bigger is better (again, I’ll pause for laughs), and conventional wisdom is wrong in thinking that if a 3-ton unit would work well, then a 4-ton central AC unit couldn’t hurt.

Here’s why: this idea was based on older central air conditioners from the 1980’s which were big, used a lot of energy and could cool the Staples Center in about 15 minutes. Remember that electricity was cheap back then, and insulation was minimal at best and nonexistent at worst.

Today, energy efficiency is the name of the game and modern central air conditioners run in cycles, slowly but steadily lowering the temperature of your house instead of rapidly.

If Your AC is Too Big – if you buy a central air conditioner that is sized too big for your house, then it will run more like that AC unit from the 80’s – it will turn on quickly, cool your house rapidly and then shut off. Your house will still be nice and cool but because it is not going through the entire cycle it was designed for, heat will soon infiltrate your house and it will have to click on again. It will begin cooling your house rapidly and then it will turn off again and again. This constant on-off cycle will defeat the purpose of its energy efficient design and drive up your electric bill.

If Your AC is Too Small – if your central air conditioner is sized too small, then it will run constantly and spend most of the day trying to catch up to where it should be. Again, your house will still be nice and cool (unless it’s several sizes too small). The problem here is that modern central air conditioners, unlike your air conditioner from the 80’s, will have a hard time cooling your entire house if they are too small because they are made to work slowly over time and don’t have the power to cool your house rapidly. They will run constantly and again run up your electric bill.

If Your AC is Sized Just Right – a properly sized central air conditioner will run through the required amount of cycles to keep your house comfortable, but will not run so much that it looses its efficiency.

Next, we are going to get into some sizing calculations, so get your nerd-hat on, and get ready to do some 4th-grade long division (or just use the calculator on your phone). Or, if you’d prefer, you can save some time and get an HVAC-Facts Report, which will do the sizing calculations for you, save you money, provide tips for your specific project, and even give you fair installation prices. Here’s an article on it:

How to Calculate the Right Sized Central Air Conditioner for Your House

Now that you know what sizes your central air conditioner comes in, you can figure out how to calculate the right sized central AC unit for your house. There are two different ways to get a ballpark size estimate for your house, and one more precise way that we will discuss in this section.

NOTE: This is for an average American home, however, and will be undersized for hot, arid climates like the Southwest. Instead of subtracting 1.0, instead subtract ZERO to get the appropriate AC size for hot and arid climates:

Method 2: Calculate the Ideal Sized Air Conditioner & Furnace for Your House

Look, you don’t want to overpay for an extra-large AC unit if you don’t need it, right? You might end up paying $800 more than you need to. And you certainly don’t want an undersized one. So, in this section, we will slow it down for a second, and take the time to calculate the ideal AC size for your home using an online, automated program.

For these calculations, we’ll draw on our own online calculator, the HVAC Design & Consultation Program. Start your calculation by entering in information about your specific home, your project, and the area in which you live. For the purpose of this example, we’ll suppose that we live in sunny (but overcrowded) Southern California:

Step 1: To Begin, Enter in Information About Where You Live, Your Home, and Your Climate:

Step 2: Continue by Answering Additional Questions:

The program will then use conditional logic to ask you additional questions it needs to give you the most accurate information, such as:

How hot does it get in your area during the summer?

How about the winter?

How much shade is usually on your house?

Do you have a basement, or a slab?

What color is you roof?

….and so on…

Questions like these must be asked when calculating the best sized air conditioner for your home. In our example home, we live in a Southern California, 1,700 sq/ft. house that gets half-covered by shade at noon, and has a basement. Again, for our example, we’ll use our own online program since it’s available on this site if you need it:

Step 3: Read the Results of Your HVAC-Facts Report

As you can see on the first page of the report below, the ideal sized air conditioner for this example home is 4-tons, and the client wouldn’t benefit much from purchasing an air conditioner higher than 14 SEER. These are ASM’s recommended size and efficiency based on the information entered into the program, but are only a fraction of the information provided by the report.

What’s also useful about this program, is that it allows you to look up any specific equipment that you might be interested in, such as a Lennox or Carrier 4-ton air conditioner, and see the exact contractor pricing for it, see what a fair installation price for your specific project would be, and avoid overpaying for an improperly-sized air conditioner. Let’s take a look:

Example Page 1 of 12 From an HVAC-Facts Report

Method 3: Calculate Central AC Size Using an AC Sizing Graph:

The second way to find out what size central air conditioner you might need for your home is to use the chart below. It is not nearly as accurate as the equation above (because it’s been around for decades), but if you’re looking for a quick ballpark value then this is worth a reference because it also takes into account the average temperatures in your region. For instance, the required size of a central air conditioner in Bozeman, Montana differs greatly from the one you’d need in our service area of Santa Clarita, California.

Start at the bottom of the graph by finding your area on the map of the continental US and note what color you are (Alaska use blue, Hawaii use orange). Then go to the top and find your zone color. Go down your zone until you find the square footage of your house, and note the required size on the left.

This is an age-old tool based on national averages. It gets an important point across regarding how size varies with temperature regions, but remember that you aren’t building an air conditioning system for “average” temperatures, you are usually building them for high temperatures, specifically for where you live; the point still remains.

As you can see, the properly sized central air conditioner for a 1,900 square foot home in Montana is quite different from the required size in Santa Clarita, California. Hopefully, this section helped you get a quick idea of what size central air conditioner is right for your home.

But keep in mind that this is just a ballpark estimate, and there are several factors that this doesn’t account for, such as how much shade you have, what kind of insulation you have, average temperature increases over the last few decades, and how high your ceilings are.

In order to account for this, you’ll need to keep reading and take a look at our next section. If used, the AC units from this graph would be undersized in Zone 1, hot and arid climates, so I would add a ton to the above results to get a closer estimate of what size air conditioner you would need in these regions. Again…it’s an estimate.

Pro Tip: Don’t Overpay for Your New Air Conditioner Installation

Look, if you’re like most of our readers, you’re doing a multi-day binge on HVAC information before you buy your new air conditioner or furnace. I get it; I’m the same way, especially when I buy a new car. After all, it’s a big investment!

Save yourself some time and money, and take a look at how to get an HVAC-Facts Report like the one that we used in the example above.It includes everything you’ve been looking for, in one easy-to-read report, including: fair installation prices, an AC sizing calculator, prices for specific air conditioners from major brands, and the list goes on and on. I won’t belabor the point. Watch this short video:

Part II: What Size Central Air Conditioner Do I Need for my House? The In-Depth Guide

If you disregarded my advice and skipped straight to this section on what size central air conditioner to buy for your home, but you still don’t have that much experience with central air conditioners or if this is all new to you, then I’d recommend that you scroll up and read Part I first.

Introduction to the “Manual-J” – The Proper Way to Calculate the Right Sized Central Air Conditioner for your Home.

If you are here to see the real deal on how your HVAC contractor should be computing a properly sized central air conditioner for your house, then prepare to nerd out!

The proper way to calculate the required size for your central air conditioner is by running a load calculation known in the HVAC industry as a “Manual-J.” (For more information on what the word HVAC means, take a look at: what does HVAC stand for?) A manual-J calculation is a detailed analysis of your home’s central air conditioning and heating needs. There are two types of manual-J calculations: a whole house load calculation and a room-by-room load calculation.

Whole house load calculation vs. room-by-room load calculation.

A whole house load calculation, or a “block calculation” is a manual-J used to determine the amount of cooling required for an entire house as a whole, and includes the heat transfer between walls, number of windows and their efficiency rating, type of insulation and how much of it, heat transfer through your concrete slab, the number of people in your house (each person gives off around 250 BTUs per hour), how many sky-lights you have, the amount of lighting you have and what type, what type of ducting and the ducting location (in your attic or inside your home’s insulation), as well as a few other factors.

A room-by-room load calculation is a manual-J that is used to calculate all of the above inputs, only it is done for each individual room and then added together as a whole. In general, a room-by-room load calculation is the best way to get an accurate idea of what size central air conditioner is best for your house, but each method is useful for different things.

When to use a house load and when to use a room-by-room load calculation.

When to use each is less a matter of science and more a matter of art. In general though, a whole house calculation is best used if your HVAC contractor is only replacing your central air conditioning unit but not your ducting, having already determined that the current ducting in your home is sufficient.

Be careful – many HVAC contractors would just assume replace your central air conditioning unit and not mess with the ducting, but the modern way of ducting a home is a lot more involved than the old way which was basically to run a tube to each room and call it good. For a contractor, it is just easier to replace the central AC unit itself and skip the ducting because the money is mostly in installing the HVAC unit itself, and it also requires the least amount of time. Ducting is a hassle and takes a decent amount of time, during which the contractor is paying his men $25-$50 an hour.

A room-by-room calculation is best if both the central air conditioner and the ducting need to be replaced. There is just too much energy (and therefore money) lost in ducting if it isn’t done properly, so make sure that you get a good contractor and do it right. There isn’t a lot of money in it, but it is an essential part of doing the job right.

Imagine, you have a room with large windows that is on the sunny side of your house. This room will heat up far more quickly than a room on the dark side of the house with no windows, right? The sunny room will probably need extra ducting and another register (an AC vent) added to ensure that it is cooled properly. If you have enough rooms like this, then you will probably need a larger size central air conditioner as well. Just something to keep in mind.

Which should you use to calculate the right size central air conditioner for your house?

I know that you aren’t going to like this answer, but to do a true manual-J calculation you will need to buy some software and I wouldn’t really recommend getting that in-depth unless you are an HVAC contractor. Remember that you are paying your contractor to come to your house and do a manual-J calculation for you! If you are absolutely determined to do it yourself, then you can purchase a pretty good Manual-J product here:

Otherwise, I would recommend two things: first, if you are going to do it yourself then start small and start with a house load calculation; and second, don’t try and tackle a full, comprehensive Manual-J by yourself. It is not an intelligence thing, it’s an experience thing so don’t let your ego get the better of you! You can mess up a lot of things in one of these calculations (I know because I’ve done it myself), all of which could lead to you buying the wrong sized central air conditioner for your house.

Don’t Overpay for Your New Air Conditioner

Look, if you’re planning on buying a new air conditioner or furnace in the next few months, then get the facts before you buy. Our HVAC-Facts Report will give you the answers you’ve been looking for in only a few minutes:

Some Additional Tips for Sizing Your Central Air Conditioner

Make your HVAC contractor do a Manual-J for your house. Remember, no HVAC contractor, no matter how experienced, can look at your house and say what size central air conditioner is best for your house. The better ones who have been doing this long enough can give you an answer after asking you some questions about your insulation and sticking their head into your attic, but be wary of anyone who looks at your house and says, “yea, you’re gonna be a 3-ton unit.” It doesn’t work that way. Just because the old unit was 3-ton doesn’t mean the new one will be!

Get multiple estimates. If you are not in Santa Clarita or the Greater Los Angeles area, then here is a good article on how to choose an HVAC contractor. Don’t skimp on your contractor – it will end up costing you more in the long run!

Consider replacing your ductwork too unless it’s in good shape. Improper ducting is like having a strong heart with clogged arteries – the system still doesn’t work. Hotter rooms should have multiple ducts and registers.

Size matters – there is a such thing as too big of an air conditioner, as well as too small. You may also try this calculator…it isn’t perfect, but will give you an idea:

Who you get to install your central air conditioner is more important than which air conditioner you buy or what size it is. See item 2 above for tips on how to choose a reputable contractor.

Once you are done with figuring out what size air conditioner is right for your house, take a look at SEER values too – here’s a good place to start: What is a Good SEER Rating?

Final Thoughts on What Size Central Air Conditioner to Buy

I hope you have found this article on what size central air conditioner to buy for your house somewhat helpful. For more answers to questions like this, visit the ASM air conditioning blog. As you get closer to buying, don't hesitate to use our central air conditioner size calculator. It will calculate the proper size, give you specific equipmet prices, and much more. Click here for more information:

The University of Virginia - 2005 / The Wharton School of Finance - 2016 / U.S. Naval Aviator 2005-2015.
At All Systems Mechanical air conditioning and heating, we believe that the experience our clients have is every bit as important as the products they receive. Simply put, our results speak for themselves, and we'd be happy to help.
If you're in the market for a new AC or furnace, make sure that you get a fair price! Try our online calculator; click the tab on the top of this page for more information.

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